Regulators are ordering some of the largest U.S. banks to find tens of billions of dollars of capital to cushion themselves in the event of a deep economic downturn.

The results of government stress tests of the ability of the 19 largest U.S. banks to weather a deep recession will be released on Thursday at 5 p.m. EDT and are expected to show about half the banks need more capital.

While the reported capital shortfalls so far are much larger than analysts had expected, bank shares soared as investors got more clarity over how well the industry will cope with perhaps the most severe recession since World War Two.

Among banks needing capital, Bank of America Corp shares closed up 17.1 percent, Citigroup Inc rose 16.6 percent and Wells Fargo & Co rose 15.6 percent. The Standard & Poor's Financials Index gained 8.1 percent.

The market likes the certainty of putting numbers on the worst-case scenarios of how much capital these banks need, said Chris Armbruster, an analyst at Al Frank Asset Management in Laguna Beach, California.

Regulators have told Bank of America it needs $34 billion, while Citigroup needs $5 billion and auto and mortgage lender GMAC LLC needs $11.5 billion, according to people familiar with the matter.

Citigroup's amount reflects its previously announced plan to convert some preferred shares into common stock. The various sources were not authorized to speak because the official stress test results are not public.

Wells Fargo needs $15 billion, Morgan Stanley needs $1.5 billion and Regions Financial Corp needs some capital, the Wall Street Journal said.

Bank of New York Mellon Corp does not need capital, a person familiar with the matter said.

American Express Co, Capital One Financial Corp, Goldman Sachs Group Inc, JPMorgan Chase & Co and MetLife Inc also do not need capital, the Journal said.

All the companies declined to comment.

In an interview on PBS' Charlie Rose Show, U.S. Treasury Secretary Timothy Geithner said none of the 19 banks are at risk of insolvency. He also said the government does not want to get involved in day-to-day management at the banks.

Geithner and the top U.S. bank regulators -- Federal Reserve Chairman Ben Bernanke, Federal Deposit Insurance Corp Chairman Sheila Bair and Comptroller of the Currency John Dugan -- will brief media on the tests at 4:15 p.m. EDT on Thursday on condition reporters withhold the information until the results are released.

Bair said on Wednesday the results should be confidence instilling.

STABILITY SOUGHT

Banks may cover any capital shortfalls through a mixture of asset sales, share sales and perhaps the conversion of preferred shares into common stock.

The government is giving banks needing capital one month to develop a plan to raise it and until November 9 to finish the job. These banks must also review their managements and board of directors to ensure proper leadership.

Banks needing capital may ask the government to issue more preferred shares, or to swap government preferred shares into convertible preferred shares. They cannot repay aid from the Troubled Asset Relief Program (TARP) until they issue debt not backed by the federal government, and for more than a 5-year term.

Calling the new capital a one-time buffer, the government pledged to stand firmly behind the banking system during this period of financial strain to ensure it can perform its key function of providing credit to households and businesses.

If banks convert preferred shares issued under TARP, the government could become one of their biggest shareholders. The White House said it will await the stress test results before commenting on banks' potential management changes.

Citigroup analyst Keith Horowitz wrote that banks, other than his own, may need to raise $75 billion after the tests.

Analysts believe other banks that may need capital include Fifth Third Bancorp, KeyCorp, PNC Financial Services Group Inc and SunTrust Banks Inc.

PRESSURE ON BANK OF AMERICA CEO

Bank of America's stress test results are certain to increase pressure on Chief Executive Kenneth Lewis, who was ousted as chairman last week in a shareholder vote.

That ouster could also lay the groundwork for his departure from his employer of 40 years, including the last eight as CEO. The largest U.S. bank has already received $45 billion of government help.

Critics fault Lewis for not backing away in December from Bank of America's planned merger with Merrill Lynch or disclosing Merrill's failing health.

The bank might raise capital by converting some of its $30 billion of privately held preferred shares into common stock, or by selling its stakes in China Construction Bank and Brazil's Itau Unibanco.

It might also sell its Columbia asset management unit, and has said it may sell its First Republic Bank business.

WELLS FARGO, CITI, GMAC

Wells Fargo opposed taking its initial $25 billion of government aid and did not get government help in buying Wachovia Corp for $12.5 billion at the year-end.

Chairman Dick Kovacevich in March called the government tests asinine. Billionaire investor Warren Buffett, whose Berkshire Hathaway Inc is the bank's largest shareholder, said on Sunday that Wells Fargo needs no more capital.

But analysts have said Wells Fargo may need a greater cushion against loan losses, despite it having taken a big writedown when it bought Wachovia.

Citigroup's capital need would be at the low end of the $5-$10 billion it had been expected to require.

The bank is preparing an exchange offer that could leave the government with a 36 percent equity stake.

GMAC, meanwhile, has been struggling with rising auto and mortgage losses, as well as falling vehicle sales at its former parent, General Motors Corp. It said on Tuesday a GM bankruptcy would not automatically trigger a GMAC filing.

(Additional reporting by Paritosh Bansal, Elinor Comlay and Dan Wilchins in New York; Mark Felsenthal, David Lawder, Glenn Somerville and Jeff Mason in Washington, D.C.; Douwe Miedema in London; and Michael Flaherty and Parvathy Ullatil in Hong Kong; Editing by Richard Chang and Ian Geoghegan)